Atomic Structure: The Atom and the Nucleus — AQA GCSE Physics
Atoms are made of a tiny nucleus surrounded by electrons. Their structure underlies radioactivity.
Structure of the atom
Atoms have a radius of about 1 × 10⁻¹⁰ m. They consist of:
- a small central nucleus containing protons (positive) and neutrons (neutral), where almost all the mass is concentrated, and
- electrons (negative) arranged in energy levels (shells) around the nucleus.
The radius of the nucleus is less than 1/10 000 of the radius of the atom.
Numbers
- Atomic number = number of protons (defines the element).
- Mass number = protons + neutrons.
- Isotopes have the same number of protons but different numbers of neutrons.
Atoms have no overall charge because protons and electrons are equal in number.
Electrons and energy levels
Electrons can move to a higher energy level if they absorb electromagnetic radiation, and fall back to a lower level by emitting radiation. If an atom loses or gains electrons it becomes a charged ion.
Exam tips
- Learn the atom's structure and the rough sizes (atom ~10⁻¹⁰ m; nucleus 10 000× smaller).
- Atomic number = protons; mass number = protons + neutrons.
- Define isotopes (same protons, different neutrons).
- Electrons absorb/emit radiation to change energy levels; ions form by losing/gaining electrons.